1. Field of the Invention
The present invention relates to a rare earth permanent magnet, in detail, relates to the rare earth permanent magnet capable to control a microstructure of an R-T-B based sintered magnet.
2. Description of the Related Art
R-T-B based rare earth permanent magnet including a tetragonal R2T14B compound as its main phase is known to show a superior magnetic characteristic, and is a representative permanent magnet with a high performance since its invention in the year 1982 (Patent Document 1). Note, “R” is a rare earth element and “T” is Fe or Fe partly substituted by Co.
R-T-B based permanent magnet, in which the rare earth element “R” is Nd, Pr, Tb, Dy or Ho, has a large anisotropic magnetic field Ha and preferable for a permanent magnet material. Among all, Nd—Fe—B based magnet, in which the rare earth element “R” is Nd, is well-balanced in saturation magnetization Is, Curie temperature Tc and anisotropic magnetic field Ha, and superior to R-T-B based rare earth permanent magnets using the other rare earth elements “R” in quantity of resources and corrosion-resistance. Thus, Nd—Fe—B based magnet is widely used.
Permanent magnet synchronous motor has been used for a power drive of consumer products, industrial machines and transportation equipment. However, permanent magnet synchronous motor in which a magnetic field of the permanent magnet is constant and an induction voltage increases in proportion to a rotational speed, and thus, driving thereof becomes difficult. Therefore, in medium and high speed ranges and under light load, a method called “a field-weakening control” came to be applied to permanent magnet synchronous motor, in order not to make induction voltage higher than the power supply voltage, making magnetic flux of the permanent magnet cancelled by a demagnetizing field due to armature current and interlinkage flux reduced. However, armature current, which does not contribute to a motor output, is continued to distribute in order to keep applying demagnetizing field. Thus, there is a problem that an efficiency of the motor is consequently reduced.
In order to solve such problems, as shown in Patent Document 2, a variable magnetic force motor using a Sm—Co based permanent magnet (a variable magnetic flux magnet) with a low coercive force which exhibits a reversible change in magnetization by applying external magnetic field has been developed. With the variable magnetic force motor, decrease in efficiency of the motor due to the conventional field-weakening control can be suppressed by reducing the magnetization of a variable magnetic flux magnet in medium and high speed ranges under light load.
With the Sm—Co based permanent magnet mentioned in Patent Document 2, however, there was a problem of being a high cost, due to expensive main materials: Sm and Co. Thus, an R-T-B based permanent magnet is applied as a permanent magnet for the variable magnetic flux magnet.
Patent Document 3 mentions the R-T-B based permanent magnet including the main phase particles having a composition of (R11-xR2x)2T14B, in which R1 is at least one kind of rare earth element not including Y, La and Ce, R2 is an rare earth element including one or more kind of Y, La and Ce, “T” is one or more kind of transition metal element and including Fe or Fe and Co as essential components, satisfying 0.1≤x ≤0.5. The R-T-B based variable magnetic flux magnet further includes 2 at % to 10 at % of “M”, in which “M” is at least one kind selected from Al, Cu, Zr, Hf and Ti. The R-T-B based variable magnetic flux magnet has a higher residual magnetic flux density relative to the conventional Sm—Co based permanent magnet for variable magnetic force motor. Thus, higher output and higher efficiency of the variable magnetic force motors are expected.
Patent Document 1: JP S59-46008A
Patent Document 2: JP 2010-34522A
Patent Document 3: JP 2015-207662A